The fleeting gold of fall

The bridge over the Fraser River just outside Kaibab Park in Granby is framed by thick golden foliage. Aspens and cottonwood trees line the Fraser River along the southern end of Granby and offer quality leaf peeping close to town.Lance Maggart / Sky-Hi News |

The temperature is dropping in the high country and throughout portions of the western Rockies autumn is in full swing with our local mountains covered in the evocative colors of fall.

The river bottoms and mountainsides of Middle Park are now awash in a vibrant cascade of gold, orange and red. Tourists are traversing the highways and byways of Grand County looking for prime leaf peeping spots, and sometimes creating traffic hazards. As you look out across the ombre’ patchwork of trees a question naturally emerges: why do some leaves and some tress change before others?

It won’t surprise most people to know there’s a scientific explanation for the process. Matt Schiltz, Forester with the Colorado State Forest Service, provided some basic details.

“The reason the aspens turn these colors is a combination of moisture, daylight time and temperature,” Schiltz said. “As the days get shorter and we get colder nights it signals to the trees to go into energy saving mode. They quit producing chlorophyll.”

As the leaves of aspen trees lose their chlorophyll the spectacular colors of fall begin to emerge. Trees do not produce the colors specifically in the fall. Rather the molecules that produce the yellows, oranges and reds of autumn are already present in the leaves but are obscured throughout the spring and summer by the heavy presence of chlorophyll, turning the leaves green.

“What you are seeing isn’t an addition of pigment,” Schiltz said. “It is a lack of chlorophyll that brings out other pigments in the leaf. Aspens have a very bright yellow color which is produced by carotenoids.” Interestingly the molecule that turns aspens yellow in fall is the same molecule that turns carrots orange.

Schiltz continued. “You’ll probably notice a lot of little pockets of aspens that may be bright orange or red or maybe even purple. That is caused by another chemical in the leaf called anthocyanin. Those molecules are naturally occurring in the leaf and as the chlorophyll dissipates those colors show through.”

Aspen leaves seem to change color the way Hemingway said people go broke, “gradually and then suddenly”. For those who call the high country home the process is ephemeral. We see the trees day after day and note the slow change, but it seems we all wake up one morning and overnight large groves have shifted from green to flaxen and the next day to gold.

The process often begins in our local river bottoms and streambeds and highlights how temperature kick starts the entire process for the trees. “The reason you see the change happen down in the valley bottoms and gradually work up the hillsides is, to some extent, the temperature,” Schiltz said. “We get cold inversions at night when all the cold air settles down into the valley bottoms. The valleys funnel the cold air. It triggers those trees to turn earlier… The temperature gradient is distinct enough in the fall to produce differences in the changes.”

Schiltz went on to explain that moisture levels are another reason trees higher up on our local mountains take longer to begin the color changing process. “This year we had a relatively dry late August and September, especially down in the valley bottoms where we don’t get as much moisture. You could potentially see the trees at higher elevations staying green longer because of the moisture as well.”

“It has a lot to do with the weather,” Schiltz said. “When are we starting to get those really cold nights and how wet was the spring summer and fall. All are factors that come into play.”

But despite having a defined set of factors that determine when and how the leaves change Schiltz pointed out there is not standardization to the process which can and does happen at different points in a given fall. “It is the culmination of all the different environmental factors that come into play,” he said. “This year, to me, it felt like it was a little bit early.”

Schiltz also said he believes environmental factors impact the vibrancy of the colors. “If it is really dry it can help to dim the fall colors and actually make it look a little more brown. But I don’t think we were that dry this year that we experienced that.”

Depending on when the color changing begins Schiltz said the process typically last between one and one-and-a-half months. “I started to notice some trees starting to turn in late August,” he said. “We will see the last of the fall colors probably around the beginning to the middle of October.”

When aspens change they often seem to change in massive groves, with small patches of green in between. Schiltz explained this results from what is called “clonal variation”. Aspen trees often exist as incredibly large colonies, with expansive root systems connect hundreds if not thousands of trees and forming them all into what is effectively a single massive organism. When you see an entire hillside of aspens all the same color, chances are they are all connected by a single root system. When you see a stand of trees that haven’t turned next to a stand that has, you can assume those are two different root systems.

This color changing process is not relegated to aspen trees alone. Several other species of trees and shrubs that are native to Grand County experience a fall color change as well, such as willows. Cottonwood trees are among the most distinguished producers of fall foliage in Grand County, besides aspens, and tend to be clustered along river bottoms and near streambeds. Cottonwoods are typically tall towering trees with columns of deep yellow, almost golden, colored leaves that billow upwards like clouds.

You may occasionally mistake aspen trees for cottonwoods from a distance but up close the difference is easy to see as cottonwoods are covered in a thick dark brown bark while aspens produce a thin sheet of pale white bark.

We encourage you to get out this weekend and enjoy the fall foliage while you can. Because, like Robert Frost said, “nothing gold can stay.”

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